1. Field of the Invention
The present invention relates to a suppressor for suppressing the background electrical conductivity of an eluate from a separation column of an ion chromatograph or for removing nontarget ions from a sample and an ion chromatograph for separation and analysis of inorganic ions or organic ions contained in a sample solution as one example of an analyzer utilizing such a suppressor.
2. Description of the Related Art
In an ion chromatograph, a sample is introduced into a separation column to separate it into component ions, and then the component ions are detected by measuring the electrical conductivity of an eluate from the separation column in an electrical conductivity measurement cell. In order to achieve high-sensitive measurement, a suppressor is provided between the separation column and the detector. The suppressor removes nontarget ions contained in an eluate from the separation column to reduce the electrical conductivity of the eluate.
A suppressor is used also in a sample pretreatment process to remove nontarget ions from a sample solution.
In a suppressor, ion exchange is performed to remove nontarget ions. Therefore, as a suppressor, one utilizing an ion-exchange column filled with an ion-exchange resin or one utilizing a planar ion-exchange membrane is used. In the case of an ion-exchange column-type suppressor, nontarget ions are removed through ion exchange by allowing a sample solution to pass through a column filled with an ion-exchange resin.
The ion-exchange column needs to be filled with a large amount of ion-exchange resin not only to increase its ion-exchange capacity but also to maintain its ion-exchange ability for a long period of time. However, filling the ion-exchange column with a large amount of ion-exchange resin leads to a larger inner diameter of the ion-exchange column than that of a tube connected to a separation column. This causes broadening of peak segments (bands), separated by the separation column, in the suppressor.
On the other hand, an ion-exchange membrane-type suppressor has two flow channels opposed to each other with an ion-exchange membrane being interposed between them. In the ion-exchange membrane-type suppressor, a sample solution is allowed to flow through one of the flow channels to remove nontarget ions through ion exchange using the ion-exchange membrane, and a regenerant is allowed to flow through the other flow channel to regenerate the ion-exchange membrane.
In the case of the ion-exchange membrane-type suppressor, its flow channel size can be made smaller as compared to the ion-exchange column-type suppressor. However, the flow channel of the ion-exchange membrane-type suppressor is rectangular in cross section, which changes flow conditions. Also in this case, broadening of peak segments occurs in the suppressor.
As a method for suppressing broadening of peak segments in a suppressor, one using an ion-exchange tube constituted by an ion-exchange membrane has been proposed (see U.S. Pat. No. 4,486,312). According to this method, an eluate from a separation column is allowed to flow inside the ion-exchange tube to perform ion exchange inside the tube to remove nontarget ions, and a regenerant is allowed to flow outside the ion-exchange tube to regenerate the ion-exchange membrane outside the tube. In this case, the inner diameter of the ion-exchange tube is made close to that of a tube connected to a separation column. This makes it possible to suppress broadening of peak segments in a suppressor.
The ion-exchange tube needs to be fixed in some way. For example, in the case of the method proposed in Patent Document 1, the tube is wound around a cylindrical core in the form of a coil, and a regenerant is allowed to flow along the wall of the coil. However, the winding of the tube around a core inevitably leads to an increase in the size of a suppressor. Further, the winding of the tube around a support such as a core makes it impossible to bring a regenerant into contact with the surface of the tube located on the support side. This reduces the regeneration efficiency of the ion-exchange membrane by a regenerant, and thus, ion-exchange performance is reduced.